The measurement of primary photosynthetic production in natural waters is of fundamental importance in ecological studies. Despite significant advances in the application of optical methods to biological oceanography, until now direct measures of primary production have been expensive, time consuming and subject to controversy. Primary production measurement by incubation of water samples inoculated with appropriate substances provides questionable results, due in part by the poor simulation of ambient light and temperature fields as they actually occur in nature.
Some of the reasons why it is important to develop inexpensive sensors for measurement of chlorophyll concentrations and primary production include the monitoring of global productivity, measuring global fisheries resources, application of above surface to underwater optical communication systems, underwater detection applications and correlation and calibration of remote sensing systems. While attempts have been made to determine primary productivity, they have been very man-power and cost intensive. The measurement of primary production is one of the most important activities of oceanographers at sea but, until now, it has been one of the most time-consuming and inaccurate of measurements. The accurate, rapid and economical measurement of primary production would aid in rapid expansion of understanding of spatial and temporal variability in production.
Measurement of light in natural waters has been conducted for many years. Recently, measurements of spectral irradiance in the ocean have indicated certain anomalies in the spectral region of 660 to 700nm (nanometers), which is the region of chlorophyll fluorescence. These anomalies have been higher than predicted when considering measurements of irradiance in this spectral region. This phenomenon has been noted but little exploited until recently. This may have been due to the specialized instruments required to accurately measure chlorophyll fluorescence in the presence of a highly variable background signal and by disagreement on its origin. One researcher first attributed the phenomenon to anomalous dispersion. A further complication to its interpretation is its dependence on a highly variable underwater light field and the complex set of factors influencing fluorescence yield.
Researchers had concentrated their measurements to the attenuation in water, according to Beers law, of the surface irradiance. Such measurements usually included the entire spectrum (400-700nm) of photosynthetically active radiation. This led first to estimates of chlorophyll concentration based on the attenuation of light by chlorophyll absorption. From these measurements of the absorption coefficient at different wavelengths, estimates based upon empirical models were made regarding primary production. Evidence of fluorescence excited by sunlight was occasionally detected but deemed to be anomalous and was ignored.
The contribution of fluorescence to upwelling irradiance in waters has been analyzed. One researcher considered Raman scattering as a possible source and rejected it. Other researchers calculated fluorescence efficiencies and chlorophyll concentrations from their measurements. It does not appear that any of them related their measurements to primary production. Before the present invention, researchers in this area did not think it possible to construct an instrument from which primary production could be determined from sensing the level of fluorescence.
Part of the problem faced by researchers in determining primary production in a parcel of water is that many pigments with different absorption spectral characteristics can couple energy to chlorophyll and on to photosynthesis. Furthermore, this mix of pigments is highly variable in nature. This can cause production estimates based on chlorophyll concentration to often give poor results. In addition, optical remote sensing methods based on light attenuation or on reflectance cannot discriminate between living organisms and non-living detritus.
Furthermore, estimates of primary production based on measurements of chlorophyll, such as those obtained from instruments with artificial excitation light sources tuned to chlorophyll, while usually giving reasonable estimates of chlorophyll, do not account for the input of photosynthetic energy through other pigment systems.